Process for making a voltage dependent resistor

ABSTRACT

A voltage dependent resistor comprising a sintered body of zinc oxide having: (1). voltage dependent properties by itself, (2). at least one member selected from the group consisting of L: ions and Na ions diffused in the side surface thereof, and (3). two electrodes applies to the opposite surfaces thereof. The invention also provides a process for making said resistor.

United States Patent 1 Masuyama et al.

[451 Sept. 18,1973

PROCESS FOR MAKING A VOLTAGE DEPENDENT RESISTOR Inventors: Takeshi Masuyama; Mikio Matsuura; Atsushi lga, all of Osaka-fu, Japan Assignee: Matsushita Electric Industrial Co.,

Ltd., Osaka, Japan Filed: Aug. 24, 1972 Appl. No.: 283,283

Foreign Application Priority Data [56] References Cited v UNITED STATES PATENTS 3,503,029 3/1970 Matsuoka 338/20 3,611,070 10/1971 Hamamoto et al. 338/20 X Primary Examiner-C. L. Albritton Attorney-E. F. Wenderoth et al.

[57] ABSTRACT A voltage dependent resistor comprising a sintered body of zinc oxide having: (1). voltage dependent properties by itself, (2). at least one member selected from the group consisting of L: ions and Na ions diffused in the side surface thereof, and (3). two electrodes applies to the opposite surfaces thereof.

The invention also provides a process for making said resistor.

12 Claims, 1 Drawing Figure PROCESS FOR MAKING A VOLTAGE DEPENDENT RESISTOR This invention relates to a preparation of a voltage dependent resistor due to the bulkthereof and more particularly a varistor comprising zinc oxide sintered body having Li ions or Na ions diffused from the side surface of the sintered body;

Various voltage dependent resistor such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltage dependent resistor are expressed by the relation: 1

l (V/C)" where V is the voltage across the resistor, I isthe current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

where V and V are voltages at a given currents I and 1 respectively. The desired value of C depends upon the kind of application to which the resistor is tobe'put. It is ordinarily desirable that the value of n be as large as possible since this exponentdetermines the degree to which the resistors depart from ohmic characteristics. Y

There have been known voltage dependent resistors comprising sintered bodies of zinc oxide with or without additives and silver paint electrodes applied thereto, as seen in the U.S. Pat. No. 3,496,512. The non-linearity of such voltage dependent resistors is attributed to the interface between the sintered body of zinc oxide with or without additives and the silver paint electrode and is controlled mainly by changing the compositions of said sintered body and silver paint electrode. Therefore, it is not easy to control the C- value over a wide range after the sintered body is prepared. Similarly, the voltage dependent resistors comprising germanium or silicon p-n junction diodes are difficult to control the C-value over a wide range because the non-linearity of these voltage dependent resistors is not attributed to the bulk but to the p-n junction. On the other hand, the silicon carbide varistors have the non-linearity due to the contacts among individual grains of silicon carbide bonded together by a ceramic binding material i.e., to the bulk and are controlled in the C-value by changing a dimension in a direction to which the current flows through the varistors. The silicon carbide varistors, however, have a rel atively low n-value ranging from 3 to 6 and are prepared by firing in non-oxidizing atmosphere, especially, in a purpose to obtain a lowerC-value. In U.S. Pat. ap plications Ser. No. 763,285 filed on Sept. 27, 1968, No. 866,820 filed on .Oct. 16, 1969, No. 866,819 filed on Oct. 16, 1969, No. 866,821 filed on Oct. 16, 1969, No.

869,470 filed on Oct. 27, 1969, No. 872,590 filed on Oct. 30, 1969, there have been disclosed voltage dependent resistors comprising sintered bodies of zinc oxide with additives such as bismuth oxide, uranium oxide, strontium oxide,'lead oxide, barium oxide, cobalt oxide and manganese oxide. The non-linearity of such voltage dependent resistors is attributed to the bulk thereof and is independent of the interface between the sintered bodies and electrodes. Therefore, it

is easy to control the C-value over a wide range by changing the thickness of sintered body itself. Such voltage dependent resistors in a bulk type have more excellent properties in n-value, transient power dissipation and AC power dissipation than SiC varistors. A disadvantage of the zinc oxide voltage-dependent resistors exists in their poor stability in an electric load life test in high humidity ambient. When DC. power is applied to the zinc oxide sintered body in a high humidity ambient, the sintered body shows a decrease in the surface electrical resistance. The decrease causes particularly an increase in the leakage current in the zinc oxide voltage-dependent resistor-of a bulk type and results in the poor non-linear property. The deterioration of the non-linear property of the voltage-dependent resistor occurs still in the load of low power such as that lower than 0.01 watt in high humidity ambient, for example 90 percent R.H at 70 C ambient. Therefore, it is necessary that the sintered body are assured completely f0 ity' ambient.

Another object of thepresent invention is to provide a method for making a voltage dependent resistor characterized'by both a high n-value and a high stability for (Le. load in high humidity ambient.

These and other objects of -the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single FIGURE is a partly cross-sectional view of a voltage-dependent resistor in accordance with the invention. I v Before proceeding with a detailed description of. the manufacturing process of the voltage-dependent resistor contemplated by the invention, the construction of the resultant resistor will be described .with reference to the aforesaid figure of drawing wherein reference character 10 .designates, as a whole, a voltagedependent resistor comprising, as its active element, a sintered body having surfaces consisting of a side surface 2 and opposite surfaces 3 and 4 to which'a pair of electrodes 5 and 6 are applied. Said sintered body 1 is prepared in a manner hereinafter set forth and have a diffusion layer of Li ions or Na ions 11 at said side surface 2 and is in any form-such as circular, square or rectangular plate form. Wire leads 8 and 9 are attached conductively to the electrodes 5 and 6, respectively, by a connection means 7 such as solder or the like.

A process for making a voltage dependent resistor characterized by a high humidity resistanceaccording to the invention comprises: f

1. providing zinc oxide sintered body having, voltage dependent properties byitself,

2. diffusing at least one member selected from the group consisting of Li ions and Na ions-into said zinc oxide sintered body from the side surface of said zinc oxide and 3. applying two electrodes to the said zinc oxide sintered body.

Said zinc oxide sintered body havng. voltage dependent properties by itself can be prepared by using a.

composition described in U.S. Pat. applications Ser.

No. 763,285, No. 866,820, No. 866,819, No. 866,821,

opposite surfaces of I tion consisting essentially of, as a major part, 80.0 to 99.9 mole percent of zinc oxide, and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi O and 0.05 to 10.00 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (C), manganese oxide (MnO), antimony oxide sb,o, barium oxide (BaO), strontium oxide (SrO)=and lead oxide (PbO).

The diffusion process referred to herein can be achieved by any suitable and available method such as firing said sintered body covered, at the side surface, with powder of lithium compound or sodium compound which is converted into lithium oxide or sodium oxide at the firing temperature. A prefereable method is to coat said sintered body with a paste including the lithium compound or the sodium compound at the side surface-and to heat at a given temperature for a given time.

An assurance of humidity stability requires a diffusion length not less than 0.01mm in accordance with the present invention. The diffusion length can be easily controlled by diffusion temperature and diffusion time in a manner per se well known in the art.The higher diffusion temperature or the longer diffusion time results in the longer diffusion length.

Among lithium ions and sodium ions, lithium ions achieve a higher stability for humidity at the same diffusion length. i

It has been discovered according to the invention that the D.C. stability of resultant resistor in high humidity is remarkably improved when said paste comprises, as a solid ingredient, 0.5 to 10.0 wt. parts of U 0 and at least one member selected from the group consisting of 0.01 to 10.0 wt. parts of C00. 0.01 to 10.0 wt. parts of MnO, 0.01 to 10.0 wt. parts of Ag O, 0.01 to 10.0 wt. parts of Cr O and 0.01 to 10.0 wt. parts of MO.

- The D.C. stability of resultant resistor is extremely improved when said paste comprises, as a solid ingredient, 0.5 to 10.0 wt. percent of lithium oxide (Li O), 3.0 to 35.0 wt. percent of boron trioxide (8,0,), 5.0 to 43.0 wt. percent of silicon dioxide (SiO and 12.0 to 91.5 wt. percent of one member selected from the group consisting of bismuth oxide (B50 and lead oxide (PbO).

According to the invention, the resultant resistor shows excellent D.C. stability in high humidity test when said paste comprises, as a solid ingredient, 0.8 to 10.0 wt. percent lithium oxide (Li,O),-50.0 to 80.0 wt. percent of barium oxide (BaO) and 10.0 to 40.0 wt. percent of boron trioxide (8,0,).

It has been discovered according to the invention that the optimal results can be obtained with the D.C. stability of the resultant resistor in humidity test when said paste comprises, as a solid ingredient, 1.0 to 2.5 wt. parts of Li o-and 1.0 to 3.0 wt. parts of K,O.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials comprising zinc oxide powder and additives such as bismuth oxide, cobalt oxide, manganese oxide, antimony oxide, barium oxide, strontium oxide, lead oxide, uranium oxide and tin oxide are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure from IOOKg/cm. to LOOOKg/cm. When the rod-shaped resistor is desired, the mixed slurry can be fabricated into the desired shape by extruding method and then dried. The pressed or extruded bodies are sintered in air at a temperature of 1,000 to 1,450 C for l to 5 hours, and then furnace-cooled to room temperature. The sintering temperature is determined from the view of electrical resistivity, nonlinearity and stability. The electrical resistivity also can be reduced by airquenching from the sintering temperature to room temperature. The mixtures may be preliminarily calcined at 700 to 1,000 C and pulverized for easy fabrication in the subsequent pressing step. The mixtures may be admixed with a suitable binder such as water, polyvinyl alcohol, etc. The said sintered body has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite surfaces. The shorter distance results in the lower C-value.

The sintered body is coated, at a side surface, with a paste including, Li oxide powder or Na oxide powder fired at a given temperature in oxidizing atmosphere so as to diffuse Li ions or Na ions into the bulk of said sintered body and then cooled to room temperature. Said paste comprises, as a solid ingredient, lithium oxide powder with or without further additives or sodium oxide powder and, as a binding material, an organic resin such as epoxy, vinyl and phenol resin in an organic solvent such as butyl acetate, toluene or the like. Said lithium oxide or sodium oxide can be replaced with any lithiumcompound or sodium compound such as oxalate, carbonate, nitrate, sulfate, iodide, bromide, fluoride, amid, hydroxyde, imide, or oxychloride which is converted into, lithium oxide or sodium oxide which diffuses easily into said sintered body as lithium ions or sodium ions at the firing temperature. The binding material is burned out during firing. The firing temperature and time depend on the weight of lithium or sodium component included in the applied paste and should be controlled so that the Li ions or Na ions diffuses into said sintered body to the depth not less than 0.01 mm. Therefore, the higher diffusion temperature requires the shorter diffusion time. The side surface layer of sintered body having Li ions or Na ions diffused therein shows very high electrical resistivity and assures a high humidity stability. The firing temperature higher than 1,000 C results in the rapid diffusion of Li ions and Na ions and makes it too difficult to control the diffusion time to a given value of the diffusion depth. On the other hand, it takes too much time to diffuse said Li ions or Na ions into said sintered body at a firing temperature lower than 600 C for Li ions and 650 C for Na ions. Therefore, diffusion temperature is more desirable to be 600-1,000 C for Li ions and 650-1,000 C for Na ions.

After diffusing process, the sintered body is applied with electrodes at the opposite surfaces of the sintered body. Said electrodes can be made by any available methodsuch as heating of noble metal paint, electroless or electrolytic plating of Ag, Cu, Ni, Sn etc., vacuum evapolating Al, Zn, Sn etc. and flame spraying of Cu, Sn, Al, Zn etc. in accordance with the prior well known technique. When said electrodes are formed by heating noble metal paint at a higher temperature than said diffusion temperature, said process of forming two electrodes is preferably carried out before said diffusing process.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder. It is convenient to employ a conductive adhesive com- EXAMPLE 2 Starting materials according to Table 2 are completed to the voltage dependent resistor and tested in the same manner as that of Example 1 except the following processes;

Pressed size: 13mm and 10mm thickness Sintering conditions: Table 2. Firing temperature and Time: 800 C for lHr. Weight of applied Li 0 or Na O: 1 mg/cm The results of tests are shown in Table 2.

Mim N.+ m oooa mJ Hi1 m so H com m6 0 0 0 3. m mm m m m h+ NH 0: mJ o v+ m omw H com m c 00o OJ m mm Mod m w 3 mac HQ H w+ w omw A com m o 0 0 m5 9mm 66 H h+ 3 m3 oJ m m+ 3 com H com m o 0 0 A6 vmm fl w o m 0mm NA m m+ m cwm H com mo o 0 0 Wed m mm E 1 5 o o 3 2 @3 5 a 2g 18 as Q3 Q6 0 t t 5 2.5 m0 unmuusu c 25 mo a o 2 E5 mo a o s 28 wovcmuudu fi gufl m m m Qqu sw m m myuq sw m m #30 5 0 m #33 sw m m #30 g a su umom. mug uouwammm umma mw uoumdmmm Bum mgen Lwmww mw wwfl owww m 8 m 6 wwmwwww wm W eflfifim wfi fiz fl flm meow oz 2 mso n fifio @33 N 3 5 m w m M w u m u m w w "m The starting materials composing of 99.0 mole percent of zinc oxide, O.5*mole percent of bismuth oxide EXAMPLE 3 and 0.5 mole percent of cobalt oxide are completed to CCSSCS.

Sintering temperature and time: l,350 C and ll-lr. Li

compound or Na compound included in the paste: Table 3,

Weight of applied Li compound or Na compound The results of test are shown in Table 3.

5 (with weight converted into U 0 and N8 0): lmg/crrr Firing temperature and time: Table 3.

Table 3.

Weight of Applied Firing EEktics h Rates after Diffusant (mg/c10 fii Fgg t-ant Life 'Dest (wt. converted into oxide 'mnperature Tme C at a glven AC(at a given 7 form) (C) (H s) current of n current of 11m) A n(%) 110A) (V) (is) 1.1 0 0.1 800 2 625 15 +4.1 -4.1 1.1 0 0.1 1000 1 635 18 1 +3.9 -4.0 Li O 0.5 700 I 1 625 16 +4.0 -4.2 1.1 0 1 600 1 630 17 +4.1 -4.1 Li O 2 800 1 635 ,17 +3.9 -4.0 Li 0 s 900 1 645 18 +4.0 -3.8 1.1 0 10 600 1 645 18 +4.0 -3.8 L1 0 10 1000 0.3 650 19 +3.9 3.8 Li 0 1 700 1 630 19 +3.8 -3.9 1.1 0 1 800 1 635 19 +3.8 -3.8 L120 1 900 1 640 .20 +3.8 -3.8 p 1.1 0 1 1000 0.5 645 20 +4.0 -4.0 1.1 0 1 800 s 640 18 +4.0 -4.1 LiCl 0.1 600 2 630 14 1 +4.2 +4.0 LiCl 1 800 1 645 17 +4.0 -4.0

LiCl 10 1000 0.3 660 20 +3.8 -3.8

Li C O 0.1 600 2 630 15 +4.0 -4.1 1.:1 c o 1 800 1 640 19 +4.0 -3.8 11 C 0 10 1000 0.3 660 18 +3.9 -4.0 1.1 00 0.1 600 2 630 19 +3.8 -3.8 Li CO 1 800 1 640 19 +4.0 -3.7 11 00 10 1000 0.3 s 18 +4.0 -3.9 11 00 0.1 600 2 630 15 +4.1 -3.9 1.1140 1 800 1 645 18 +3.9 -3.8 1.1140 10 1000 0.3 660 20 +3.7 ,-3.8 1.1 0.1 900 2 630 16 +4.2 -4.2 1.1 80 1 900 1 640 19 +4.1 -4.0 1.1 50 10 1000 0.3 665 20 +4.0 -4.1 LiI 0.1 600 2 635 17 +4.1 -4.1

L11 1 800 1 640 17 +3.8 -4.1 1.11 10 1000 0.3 665 20 +4.0 -3.5 LiBr 0.1 600 2 635 18 +4.0 +4.1. L151: 1 00 1 640 18 +3.9 -4.0 LiBr 10 1000 0.3 660 19 +4.0 -3.7

LiF 0.1 800 2 630 15 +4.0 -4'.2

w M a m mtm om %,M H t( 2m Mm.. n m m m C at a 9 (H Current Of lmA) Diffusant 9 (wt. om'xw into oxide mperatum form) Li NH Li Ll NH LiClO Liclo l O 8 9 8 0 0 o 0 0 8 8 7 7 7 8 5 7 0 5. u 0 I 0 0 6 6 6 7 6 6 7 7 7 8 l l l l l 5 o 0 0 5 2 3 A. 4 4 6 6 6 6 6 3 l l l 1 0 0 0 0 0 0 5 0 0 0 0 6 8 9 8, O l l 2 5 0 0. 1 1

0 O O 0 O 2 2 2 2 2 a a a a a N N N N N 00 O I 8 76 l I 6 97 l 05 43 6B 5 11 O0 2 2 a a N N NaBr NaBr

NaBr

NaI

' NaI NaI 

2. A process for making a voltage dependent resistor comprising zinc oxide sintered body having voltage dependent properties by itself, said process comprising: (1.) providing zinc oxide sintered body having voltage dependent properties by itself, (2) diffusing Li ions into said zinc oxide sintered body from the side surface of said zinc oxide sintered body, and (3.) applying two electrodes to the opposite surfaces of said zinc oxide sintered body.
 3. A process according to claim 1, in which said zinc oxide sintered body consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide, and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi2O3) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO), antimony oxide (Sb2O3), barium oxide (BaO), strontium oxiDe (SrO) and lead oxide (PbO).
 4. A process according to claim 1, in which said Li ions or Na ions are diffused into said sintered body to the depth not less than 0.01 mm from the side surface of said zinc oxide sintered body.
 5. A process according to claim 2, in which said Li ions are diffused by applying a paste comprises, as a solid ingredient, 0.5 to 10.0 wt. parts of Li2O and at least one member selected from the group consisting of 0.01 to 10.0 wt. parts of CoO, 0.01 to 10.0 wt. parts of MnO, 0.01 to 10.0 wt. parts of Ag2O, 0.01 to 10.0 wt. parts of Cr2O3 and 0.01 to 10.0 wt. parts of NiO.
 6. A process according to claim 5, in which said paste comprises, as a solid ingredient, 0.5 to 10.0 wt. percent of lithium oxide (Li2O), 3.0 to 35.0 wt. percent of boron trioxide (B2O3), 5.0 to 43.0 wt. percent of silicon dioxide (SiO2) and 12.0 to 91.5 wt. percent of one member selected from the group consisting of bismuth oxide (Bi2O3) and lead oxide (PbO).
 7. A process according to claim 5, in which said paste comprises, as a solid ingredient, 0.8 to 10.0 wt. percent of lithium oxide (Li2O), 50.0 to 80.0 wt. percent of barium oxide (BaO) and 10.0 to 40.0 wt. percent of boron trioxide (B2O3).
 8. A process according to claim 5, in which said paste comprises, as a solid ingredient, 1.0 to 2.5 wt. parts of Li2O and 1.0 to 3.0 wt. parts of K2O.
 9. A voltage dependent resistor comprising zinc oxide sintered body having: (1.) voltage dependent properties by itself, (2.) at least one member selected from the group consisting of Li ions and Na ions diffused in the side surface thereof, and (3.) two electrodes applied to the opposite surfaces thereof.
 10. A voltage dependent resistor comprising zinc oxide sintered body having: (1.) voltage dependent properties by itself, (2.) Li ions diffused in the side surface thereof, and (3.) two electrodes applied to the opposite surfaces thereof.
 11. A voltage dependent resistor accroding to claim 9, in which said zinc oxide sintered body consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide, and, as an additive, 0.05 to 10.0 mole percnet of bismuth oxide (Bi2O3) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO), antimony oxide (Sb2O3), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).
 12. A voltage dependent resistor according to claim 9, in which the depth of said Li ions or Na ions diffused in said zinc oxide sintered body is not less than 0.01 mm from the side surface of said zinc oxide sintered body. 